This invention relates to platey alumina, and particularly to platey alumina useful as a lapping powder.
"Lapping" is a finishing technique that is usually performed on a single or double sided lapping machine with abrasive provided in the form of loose particles suspended in a slurry and delivered to the surface of a part to be lapped. The part is held in carriers on or between lapping plates which are usually made of cast iron.
Properly executed lapping performs four functions:
1. It reduces a part to an accurate and uniform thickness; PA1 2. It produces a part with essentially flat and parallel surfaces; PA1 3. It removes surface and subsurface defects resulting from prior finishing operations such as sawing or cutting, providing these are not too deep; and PA1 4. it performs the above functions while leaving minimum surface finish and depth of sub-surface damage. PA1 a. providing a platey alumina product containing from about 10 to about 30% by weight of aggregates; PA1 b. classifying the platey alumina product into a coarse fraction and a fine fraction containing particles smaller than 6 microns; PA1 c. wet milling the coarse fraction until the particle size of the powder has reached a D.sub.50 of from about 7 to about 15; and PA1 d. separating and removing a fraction with a particle size less than 3 and preferably less than about 6 microns.
The lapping process requires an abrasive that is of extreme uniformity in terms of particle size so as to avoid scratching the lapped surface. Over the years it has become a common practice to use platey alumina for this application.
Platey alumina is characterized by the shape of the particles which are comparable to hexagonal tiles. The individual particles have an aspect ratio, defined as the largest dimension divided by the smallest dimension, of at least about 10:1. The particle size is defined by standard measuring techniques such as by use of a Coulter Multisizer counter.
Platey alumina is produced commercially f or example by Alcoa, (sold as P-25 alumina), and Lonza, (sold as MNY alumina) among others by the controlled calcination of alumina trihydrate. The product is often called "calcined alumina" and, as produced, conventionally contains significant quantities, (sometimes up to 100% of the weight of the powder), of agglomerates in which the platelets are strongly fused together. Clearly the use of such as-fired platey alumina as a lapping powder would be very inappropriate for the production of a high quality surface because the agglomerates would tend to scratch the surface and/or cause significant subsurface damage. Therefore manufacturers of lapping powders subject the platey alumina as purchased to prolonged milling, using non-metallic media, to break up the aggregates. The seminal patent in this field is U.S. Pat. No. 3,121,623 which sets forth the separation techniques that have become conventional. The product of this milling process is then subjected to hydraulic elutriation to obtain the final lapping powder.
The platey aluminas that are the subject of this invention are primarily of interest in lapping expensive materials such as silicon wafers. As indicated above, lapping is an essential step in the production of the substrates from which microchips are produced and the lapped wafer must be free from scratches and subsurface damage if the chips produced are to be useful. Scratches are defined as elongated gouges in the surface and are relatively easily detected by the microscope or even the unaided eye when they are particularly deep. They are commonly defined as those scratches that are visible to the naked eye after etching of the the surface. Subsurface damage, (SSD), is more difficult to detect and may comprise pitting or other surface cratering. It is not usually detectable until the lapped surface has been etched. The purpose of polishing is to remove SSD by removing surface material to a depth below the damage depth. This however must be kept to a minimum because the process is slow and primarily concerned with leaving a uniform flat surface without causing further SSD. In addition, since only one surface is polished at a time, there is a potential for leaving a surface that has diminished parallelism with the opposed surface.
The milling operation described in U.S. Pat. No. 3,121,623 takes several hours, (typically five to eight hours), to complete and results in a product in which the platelets are essentially completely separated but in which there are significant quantities of broken platelets. These can lead to SSD when used to lap and the best grades of platey alumina try to minimize their occurrence.
Another source of SSD is the presence of fines which tend to make the platelets lie unevenly such that one edge is raised. This can cause scratching of the lapped surface. The elutriation described in the above patent eliminates much, but not all, of the fines produced during the milling process.
As a response to the above, great efforts are made to reduce the amounts of fines in the best quality lapping powders. Broken platelets have not been easy to control and have largely been accepted as an unavoidable fact of life in a commercial process.
In a typical commercial process an as-fired, agglomerated platey alumina feed is wet milled in a Sweco mill for 5-8 hours till the desired D.sub.50 size distribution is obtained and the mixture is then subjected to repeated decanting and settling operations to take out fines. The resulting slurry is then dried and pin-milled to break up any loose agglomerates formed during drying, classified, bagged and shipped. The operation takes from about 20 to 25 hours and results in a saleable yield of about 55% of the theoretical.
Such performance is clearly not very attractive from an economic standpoint. The yield is low and the time taken to achieve a quality saleable product is too long.
The present invention provides a way of obtaining a platey alumina lapping powder superior in quality to anything obtained before, using a process in which the time-in-process is very significantly reduced and the yield is significantly increased. The new product is also more uniform than any previously available platey alumina lapping powder, consists essentially of euhedral platelets and, when used in lapping silicon wafers, reduces the incidence of scratching and SSD to a level hitherto unknown.